Fuel injection systems are widely used in internal combustion engines. These fuel injection systems typically employ one or more solenoid operated fuel injectors that provide fuel to a corresponding combustion chamber at the appropriate time during the combustion cycle of the internal combustion engine. By controlling the amount of fuel introduced into the combustion chamber, the overall operation of the engine can be more effectively controlled.
Electronic control systems are used for controlling the timing, the waveshape, amplitude, and the duration of the signals which actuate the fuel injectors. The electronic control system usually provides a relatively high amplitude current to the fuel injectors during an initial or pull-in period of time to achieve an initial actuation of the solenoid operated fuel injectors. Subsequently, a smaller magnitude current is provided to the solenoid operated fuel injectors to maintain actuation of the solenoid during a subsequent holding or hold period of time. As a result of this "peak and sustain" or "peak and hold" control, the energy required to operate the fuel injector solenoid is usually increased since the relatively high amplitude current is initially permitted to ensure a fast actuation response of the solenoid while the smaller magnitude current is utilized to maintain the actuation of the solenoid with less power dissipated. These electronic control systems typically have a reliable and stable source of power (i.e., an alternator/battery system of a vehicle) that applies energy to the solenoids of the fuel injectors. When energy is initially applied to the fuel injectors, current flow through the fuel injectors will build up gradually due to the inductance of the solenoids. However, the mechanics of the fuel injectors will not actuate until the current reaches a certain threshold which depends on the characteristics of the electromagnetic and mechanical properties of the actuators. The amount of time for the current to build up to actuate the fuel injectors is dependent upon the voltage applied to the fuel injector.
In most vehicles, the power supplied to the fuel injectors is provided by a battery, usually producing a nominal 12 volt DC signal. However, the voltage of the battery can vary substantially during normal operation of the vehicle. When the battery voltage is low, it will take longer for the current in the fuel injectors to build up than when the battery voltage is high. In addition, under certain conditions, the battery voltage may drop to an inadequately low level which may not build up or maintain adequate current in the solenoids of the fuel injectors.
In some prior art systems, in addition to utilizing power from a battery, a high voltage power supply, known as a "boost" power supply, has been applied to the fuel injectors at the beginning of the pull-in period of time to overcome the solenoid inductance and the inertia of the fuel injector solenoid mechanics. Typically, this boost power supply includes a large reservoir capacitor to provide high peak power. The boost power supply can build up solenoid current quickly to insure rapid turn-on of the fuel injectors in response to a control signal. The boost power supply is usually only driving the fuel injector solenoid at the beginning of the pull-in period of time.
However, in some applications, where operation is required at very low battery voltages, it may be necessary to apply the boost power supply at instants other than just during the initial current rise time. Furthermore, in some applications, a rapid succession of boost power supply sourced pull-in pulses may be required, thus significantly increasing the boost power supply's power rating, and the recovery rate of the reservoir capacitor between rise times. Typically, the boost power supply is designed oversized in order to meet these requirements. In addition, the power dissipated by the boost power supply is significant and proportional to the boost power supply's power rating, thus it is desirable to appropriately size the boost power supply by minimizing a proportion of energy delivered by the boost power supply and to maximize a proportion of energy delivered by the battery supply to the fuel injector solenoid.
What is needed is an improved approach for driving loads, solenoids in particular, that will operate under low battery voltage to supplement the drive provided by the battery.